1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits.
2. Description of Related Art
There are some power conversion circuits which accomplish power conversion from one DC voltage level to another DC voltage level using an intermediate energy storage mechanism such as a power inductor. Examples of such power conversion circuits include the buck, boost, and buck boost (flyback) converters which are well known to those skilled in the art of power conversion. These converters are simple, requiring only one large magnetic storage element, the power inductor, and two switches. Typically input and output capacitors are added to the basic circuit. For isolated power conversion an isolation transformer or a coupled inductor is required in addition to or instead of the power inductor. One problem associated with practical non-isolated power conversion circuits that employ the buck, boost, or buck boost (flyback) converters is that either the input current, the output current, or both the input and output current are pulsating, that is discontinuous. The pulsating currents result in high ripple currents and high ripple voltages by comparison to inputs or outputs in which the input or output current is continuous or non-pulsating. In order to reduce the effects of the pulsating currents an additional filter stage using an additional inductor and capacitor, or LC combination, or the use of a much larger input and/or output capacitor is required. The consequence of the additional filtering is higher cost and, in the case of the LC filter, higher losses. For example, the buck converter has a pulsating (discontinuous) input current and a non-pulsating (continuous) output current. Power supplies that use buck converters often employ an LC input filter to reduce the ripple resulting from the pulsating input current. One specific example is the Datel dual output BMP models which use a buck post regulator with an LC input filter to generate a second power supply output. The BMP models employ the main output of the power supply as the input to the buck post regulator. If they did not use an input LC filter with the buck post regulator the pulsating input current of the buck post regulator would compromise the ripple performance of the converter's main output. One example of prior art that overcomes the problem is the famous Cuk converter, shown in FIG. 1. The Cuk converter circuit is optimal in the sense that both the input current and output current are non-pulsating. One of the likely reasons that the Cuk converter was not chosen in the Datel design is that the non-isolated Cuk converter has an output voltage that is opposite in sign to its input. Another shortcoming of the Cuk converter is that its control to output transfer function has a right half plane zero which, in practical terms, limits the available bandwidth, which in some cases, especially where fast load current transient response is a requirement, renders the Cuk converter unsuitable. Cuk, in a paper included in his book entitled "Advances in Switched-Mode Power Conversion" on page 319 explains how the re-orientation or rotation of the switching cell in the Cuk converter leads to buck and boost converters with continuous input and output currents. The continuous input and output current buck and boost converters that Cuk describes in his book on page 319 are simply the well known buck converter with an LC input filter and the well known boost converter with an LC output filter, respectively. The Datel design is then the buck implementation of the Cuk converter as described by Cuk in his own writings. The same process of generating continuous input and output current buck and boost converters from the Cuk switching cell is also described in a recent book by Robert Erickson entitled "Fundamentals of Power Electronics" on pages 141 and 142. In Erickson's book he describes a process for generating the buck and boost forms of the Cuk converter similar to the process described by Cuk with identical results. There is one important feature of the Cuk converter which is missing in the buck and boost derivations described by Cuk and Erickson. The Cuk described buck and boost derivations both require two separate inductors which cannot be magnetically coupled. This requirement is due to the fact that the AC wave forms of the two inductors are dissimilar, whereas the AC wave forms of the two inductors in the Cuk converter are very similar or identical at all times which enables the combination of the two chokes of the Cuk converter in a single simple magnetically coupled inductor construction. In general two inductors in the same circuit can be coupled on a single magnetic core if the ratio of the winding voltages are always proportional or nearly proportional to the ratio of the turns of the two inductors. In the non-pulsating buck and boost converters described by Cuk and Erickson one of the chokes has a large AC winding voltage component and the other has a very small AC winding voltage component. In the same book by Cuk on page 337 Cuk describes how his optimal topology converter works when implemented with the two chokes combined in a single simple coupled inductor, as illustrated in FIG. 1. Several patents have been issued on the basic Cuk converter and the coupled inductor versions of the Cuk converter. Several patents have been issued on zero ripple variations of the Cuk converter. These patents describe simple methods to reduce or null the ripple at one terminal and more complex means to achieve zero ripple at both input and output terminals. What is needed are simple optimal topology converters in which the two inductors can be combined into a single simple magnetically coupled inductor and can accomplish step up or step down DC to DC conversion without inversion or isolating transformer. What is also needed is a less complex converter topology that can achieve zero ripple at both input and output terminals. By achieving zero ripple at both input and output terminals one can reduce the inductance of the coupled inductor and achieve superior transient response, lower cost, and lower EMI simultaneously. Such an optimal topology converter in the buck form would reduce the size and cost of converters of the Datel BMP type by combining the two inductors into a smaller simple single coupled magnetic circuit element. Such an optimal topology converter in the boost form would satisfy a need for converters similar to the Datel BMP type, but where the main output voltage is lower than the voltage of the second output. The boost form would also find application in power factor correction circuits.
Consider the Cuk converter of FIG. 2. Here we emphasize that the Cuk converter can be considered to be a three terminal network. The fact that the input terminal contains a series inductor suggests that the Cuk converter's input terminal current will be non-pulsating. The output terminal also contains a series inductor so the output terminal current of the Cuk converter will also be non-pulsating. FIG. 3 also illustrates a three terminal network. Let the FIG. 3 network be a single pole double throw (SPDT) switch which is pulse width modulated (PWM). We will assume that either S1 is closed or S2 is closed but not both at the same time. We can develop a simple relationship between the terminal voltages which we will call the unified PWM SPDT transfer function. This relationship is EQU V.sub.C =V.sub.B +D.multidot.(V.sub.A -V.sub.B) (1)
where V.sub.A is the average A terminal voltage, V.sub.B is the average B terminal voltage, V.sub.C is the average C terminal voltage, and D is the duty cycle of the S1 switch. Suppose that there are voltage sources connected to both the A and B terminals of FIG. 3. The voltage at the C terminal will be a square wave whose average voltage is given by equation (1), but which has a large AC component. If we wanted to attach a load at the C terminal that required a DC voltage we could filter the C terminal voltage. A capacitor connected to ground provides a filtering function that will reduce the AC component, but placing a capacitor connected to ground will create higher currents and high stress in real world S1 and S2 switches. To avoid this problem power conversion engineers place an inductor in the C terminal, as illustrated in FIG. 4. The FIG. 4 circuit provides a basic three terminal PWM conversion network. The equation (1) still applies to the average terminal voltages because the addition of the inductor does nothing to change the average terminal voltages. An ideal inductor is a DC short circuit and an AC impedance so that the DC voltage on either side of the inductor in FIG. 4 is the same.
By connecting the A terminal to an input DC source V.sub.IN, connecting the B terminal to ground or zero volts, and connecting the C terminal to a load whose voltage is V.sub.OUT we create a buck converter and the unified PWM SPDT transfer function, equation (1), reduces to V.sub.OUT =D.multidot.V.sub.IN, which is the well known transfer function for a buck converter. Alternately, if we connect the input DC source to the C terminal, ground to the B terminal, and connect the load to the A terminal then the unified PWM SPDT transfer function reduces to ##EQU1##
where D' is the duty cycle of the S2 switch and D+D'=1. Alternately, if we connect the A terminal to the input DC source, the C terminal to ground, and the B terminal to the load, applying the unified PWM SPDT transfer function yields the result ##EQU2##
which is the well known result for the buck boost (flyback) converter. We have shown that a single unified PWM SPDT transfer function applies to all three basic converter types when applied to a generic PWM SPDT three terminal conversion network.
In the parent patent application it was shown that the FIG. 5 circuit, a variation of the FIG. 4 circuit, can yield converters with buck and boost transfer functions. FIG. 5 represents another general PWM SPDT power conversion network to which the unified PWM SPDT transfer function applies. Consider the average voltage at the terminals of the C1 capacitor. The positive terminal is connected through an inductor to the A terminal. The average or DC voltage across L_A will be zero so the average voltage at the positive terminal of C1 will be V.sub.A. Similarly the average voltage at the negative terminal will be V.sub.B because the negative terminal of C1 is connected to the B terminal through an inductor whose DC voltage is zero. The average terminal voltages of our SPDT switch are the same in the FIG. 5 circuit as they are in the FIG. 4 circuit. The buck boost (flyback) form of the FIG. 5 circuit is the well known Cuk converter.